Permanent mold casting has been employed in the past as a relative low cost casting technique to mass produce aluminum, copper, and iron based castings having complex, near net shape configurations. Only fairly recently have attempts been made to produce titanium and titanium based alloy castings using permanent mold casting. For example, the Mae et al. U.S. Pat. No. 5,119,865 issued Jun. 9, 1992, discloses a copper alloy mold assembly for use in the permanent mold, centrifugal casting of titanium and titanium based alloys.
The Colvin U.S. Pat. No. 5,287,910 of common assignee herewith describes casting of reactive metals and alloys such as titanium, titanium alloys, and nickel based superalloys in reusable, metallic molds or dies made of iron based and/or titanium based alloys and having certain mold body-to-mold cavity volume ratios.
In the casting of titanium-aluminum alloys in metallic molds, the inventors have discovered that certain alloys having aluminum concentrations generally exceeding about 10 weight % aluminum result in the deposition of an aluminum layer on the metallic mold surfaces relatively quickly as successive alloy charges are cast in the mold under a relative high vacuum, such as less than 100 microns (0.1 Torr). The aluminum layer was found to adversely affect the surface quality of successive castings made in the mold. In particular, the deposited aluminum layer produced a rough as-cast surface finish on the castings and eventually caused subsequent castings to adhere to the mold after several alloy charges were introduced into the mold.
An object of the invention is to provide method and apparatus for casting titanium-aluminum alloys in a metallic mold or die in a manner that retards or avoids deposition of the aforementioned deleterious aluminum layer on the mold or die cavity surfaces.
Another object of the invention is to provide method for casting titanium-aluminum alloys in a metallic mold or die wherein the deleterious aluminum layer is periodically removed from the mold or die surfaces.